(1) Field of the Invention
The present invention relates to a container for use in heating by a microwave oven. The container is particularly adapted for use in cooking (i.e., baking or steaming) foods such as bread, cake and the like.
(2) Description of the Prior Art
Microwave ovens are widely used in commercial and industrial fields as well as in homes for various purposes including the heating and cooking of foods, since, with the microwave oven, a container having a lower heat resistance can be used than those required for heating by a gas range or oven, the time required for heating is very short, the contents are not burned, and operation is simple. Various types of instant foods have, thereofore, been marketed which are capable of being reheated or cooked by a microwave oven.
However, in heating by a microwave oven, the degree of irradiation with microwaves varies at different parts of the food being heated so that, for example, the upper surface and corners of the food may be heated by greater irradiation of microwaves than other parts, while other parts, for example, the bottom of the food, may be insufficiently heated because of receiving little irradiation by microwaves. As a result, the food is not heated uniformly. In particular, where bread or cake are baked or steamed by a microwave oven, uniform heating is hindered convection of the contents does not occur. In addition, since an oblong container is usually used for bread or cake, the portion of the material near the side wall on the short side of the container is over-heated so that water included in such portion is vaporized. As a result, this portion becomes hard and dry. The resulting cake or bread has poor taste and appearance. Accordingly, when the bread or cake is cut along a short side in the usual method, the texture, taste and appearance will vary within each slice. As the portion of the cake near the top of the container proper is also over-heated, the degree of heating is also different between the top and bottom portions of the slice. At the same tiime, the degree of rising of the bread or cake is low because the portion near the top of the container proper is hardened by heating before the content has sufficiently risen. As a result, the conventional cooking method is not good, particularly whe applied to baking or steaming breads and cakes.
Therefore, various improvements for inhibiting local over-heating in microwave oven cooking have been devised. For example, Japanese Utility Model Publication No. 2362/1976 discloses a container for a Japanese dish known as "chawanmushi" in which a metallic film is provided either on the external surface or on the internal surface of a lid so as to reflect microwaves, and in which a resistant film is also provided on the bottom of the container for absorbing microwaves. However, the container has a problem in that its side wall becomes over-heated when the container is heated for a long time during cooking.
Japanese Utility Model Public Disclosure (KOKAI) NO. 189795/1983 and Japanese Patent Public Disclosure (KOKAI) No. 32261/1981 also propose a food package having a microwave shielding member provided on the side surface of a package. This package is effective in holding down over-heating of the side wall. On the other hand, however, over-heating of the upper portion thereof cannot be inhibited and, moreover, the heating efficiency is lowered.
Furthermore, the specification of Japanese Patent Public Disclosure (KOKAI) No. 126743/1979 discloses covering the periphery of food with a metallic foil selected in accordance with the speed at which the food is to be heated, but it does not disclose a means elevating the container proper above the microwave oven table. Accordingly, the temperature of the content locally rises, and there is a possibility of sparking during cooking in a microwave oven. Accordingly, the above-mentioned method is not appropriate for baking or steaming cake or bread.